ORGANIC LETTERS
Asymmetrically 4,7-Disubstituted Benzothiadiazoles as Efficient Non-doped Solution-Processable Green Fluorescent Emitters
2009 Vol. 11, No. 22 5318-5321
Yuan Li,† Ai-Yuan Li,† Bi-Xin Li,‡ Ju Huang,† Li Zhao,† Bao-Zheng Wang,† Jun-Wen Li,† Xu-Hui Zhu,*,† Junbiao Peng,† Yong Cao,† Dong-Ge Ma,‡ and Jean Roncali*,§ Institute of Polymer Optoelectronic Materials and DeVices and Key Laboratory of Special Functional Materials of Ministry of Education, South China UniVersity of Technology, Guangzhou 510640, China, State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, CAS, Changchun 130022, China, and Linear Conjugated Systems Group, CNRS, CIMA, UniVersity of Angers, 2 Bd LaVoisier, Angers F-49045, France
[email protected];
[email protected] Received September 30, 2009
ABSTRACT
Asymmetrically 4,7-disubstituted benzothiadiazole derivatives involving a carbazolyl moiety at one end and a solubilizing dendron at the opposite end have been synthesized and characterized. A two-layer electroluminescent device based on one of these solution-processed molecular emitters revealed a maximal luminous efficiency of ∼10.6 cd A-1 and green light emission with CIE coordinates (0.34, 0.58).
Solution-processable molecular semiconductors have received increasing attention as active materials in organic optoelectronic devices such as organic light-emitting diodes (OLEDs),1,2 solid-state lasers,3 field effect transistors (OFETs),4 and organic solar cells (OSCs).5 With respect to their polymer counterparts, small molecular active compounds present the advantages of a monodisperse unequivo†
South China University of Techonology. Changchun Institute of Applied Chemistry. University of Angers. (1) Lo, S. C.; Burn, P. L. Chem. ReV. 2007, 107, 1097. ‡ §
10.1021/ol9022563 CCC: $40.75 Published on Web 10/23/2009
2009 American Chemical Society
cal chemical structure and thus of a potentially better reproducibility of synthesis and purification and more (2) (a) Yang, R. Q.; Xu, Y. H.; Dang, X. D.; Nguyen, T. Q.; Cao, Y.; Bazan, G. C. J. Am. Chem. Soc. 2008, 130, 3282. (b) Liu, G.; Li, A.-Y.; An, D.; Wu, H.-B.; Zhu, X.-H.; Li, Y.; Miao, X.-Y.; Deng, W.-L.; Yang, W.; Cao, Y.; Roncali, J. Macromol. Rapid Commun. 2009, 30, 1484. (3) (a) Samuel, I. D. W.; Turnbull, G. A. Chem. ReV. 2007, 107, 1272. (b) Lai, W. Y.; Xia, R. D.; He, Q. Y.; Levermore, P. A.; Huang, W.; Bradley, D. D. C. AdV. Mater. 2009, 21, 355. (c) Xia, R. D.; Lai, W. Y.; Levermore, P. A.; Huang, W.; Bradley, D. D. C. AdV. Funct. Mater. 2009, 19, 2844. (d) Huang, J.; Liu, Q.; Zou, J. H.; Zhu, X.-H.; Li, A.-Y.; Li, J.-W.; Wu, S.; Peng, J. B.; Cao, Y.; Xia, R.; Bradley, D. D. C.; Roncali, J. AdV. Funct. Mater. 2009, 19, 2978.
straightforward analysis of structure-properties relationships. Furthermore, when adequately solubilized by design these solution-processable molecular materials afford the possibility of low-cost large area electronics. The current development of solution-processed small molecular RGB emitters in OLEDs, particularly for display purpose, requires highly efficient light-emitting materials with enhanced electron injection and transport properties.6,7 Although a number of emitters with predominant hole transporting properties are available, their wide applicability as a bulk emitter or dopant in a hole-transporting host might present a potential challenge as it implies the use of hole blockers combining sufficient device durability and convenient synthetic accessibility.8 We have recently reported soluble red fluorescent molecular glasses based on dithienylbenzothiadiazole combining high solubility and excellent solution processability with intrinsic morphological stability.3c,9 Notably, one of these pure red emitters has demonstrated hole injection and electron transporting properties,9b unlike many other red-emitting materials that are largely dominated by hole transport.10,11 In this context, we present here new efficient non-doped solution-processable green electroluminescent molecular emitters based on asymmetrically 4,7-disubstituted benzothiadiazoles 1a and 1b (Scheme 1). A solubilizing dendron was attached at one side of 2,1,3benzothiadiazole of compound 1a to provide the desired solubility. A 3,6-di(1-naphthyl)carbazolyl moiety12 was introduced at the opposite side of the molecule through a fluorenyl spacer in order to facilitate hole injection and glass formation. Overall the branched structures are expected to afford green emission with a potentially high photoluminescent (PL) efficiency and high color purity in the solid state by suppressing strong intermolecular interactions generally (4) (a) Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14, 99. (b) Allard, S.; Forster, M.; Souharce, B.; Thiem, H.; Scherf, U. Angew. Chem., Int. Ed. 2008, 47, 4070. (c) Sirringhaus, H. Proc. IEEE 2009, 97, 1570. (5) (a) Llloyd, M. T.; Anthony, J. E.; Malliaras, G. G. Mater. Today 2007, 10, 36. (b) Roncali, J.; Leriche, P.; Cravino, A. AdV. Mater. 2007, 19, 2045. (c) Roncali, J. Acc. Chem. Res. 2009; DOI: 10.1021/ar900041b. (d) Jørgensen, M.; Krebs, F. C. J. Org. Chem. 2005, 70, 6004. (6) Kulkarni, A. P.; Tonzola, C. J.; Babel, A.; Jenekhe, S. A. Chem. Mater. 2004, 16, 4556 Special issue on organic electronics. (7) (a) Chen, A. C. A.; Culligan, S. W.; Geng, Y.-H.; Chen, S.-H.; Klubek, K. P. K.; Vaeth, M.; Tang, C. W. AdV. Mater. 2004, 16, 783. (b) Lai, W. Y.; Zhu, R.; Fan, Q. L.; Hou, L. T.; Cao, Y.; Huang, W. Macromolecules 2006, 39, 3707. (c) Wang, L.; Jiang, Y.; Luo, J.; Zhou, Y.; Zhou, J.-H.; Wang, J.; Pei, J.; Cao, Y. AdV. Mater. 2009; DOI: 10.1002/ adma.200901039. (8) Kwong, R. C.; Nugent, M. R.; Michalski, L.; Ngo, T.; Rajan, K.; Tung, Y. J.; Weaver, M. S.; Zhou, T. X.; Hack, M.; Thompson, M. E.; Forrest, S. R.; Brown, J. J. Appl. Phys. Lett. 2002, 81, 162. (9) (a) Huang, J.; Li, C.; Xia, Y. J.; Zhu, X. H.; Peng, J. B.; Cao, Y. J. Org. Chem. 2007, 72, 8580. (b) Huang, J.; Qiao, X. F.; Xia, Y. J.; Zhu, X. H.; Ma, D. G.; Cao, Y.; Roncali, J. AdV. Mater. 2008, 20, 4172. (10) Review on red electroluminescent materials: Chen, C. T. Chem. Mater. 2004, 16, 4389, and references therein. (11) For instance: (a) Thomas, K. R. J.; Velusamy, M.; Lin, J. T.; Sun, S. S.; Tao, Y.-T.; Chuen, C.-H. Chem. Commun. 2004, 2328. (b) Li, Z. H.; Wong, M. S.; Fukutani, H.; Tao, Y. Chem. Mater. 2005, 17, 5032. (c) Chen, C.-T.; Wei, Y.; Lin, J.-S.; Moturu, M. V. R. K.; Chao, W.-S.; Tao, Y.-T.; Chien, C.-H. J. Am. Chem. Soc. 2006, 128, 10992. (d) Lee, Y.-T.; Chiang, C.-L.; Chen, C.-T. Chem. Commun. 2008, 217. (12) Zhao, L.; Zou, J. H.; Huang, J.; Li, C.; Zhang, Y.; Sun, C.; Zhu, X. H.; Peng, J. B.; Cao, Y.; Roncali, J. Org. Electron. 2008, 9, 649. Org. Lett., Vol. 11, No. 22, 2009
Scheme 1. Synthetic Routes to Compounds 1a/1b
associated with donor-acceptor molecules. For compound 1b the introduction of a trifluoromethyl group onto the carbazolyl ring is expected to further modulate charge injection/transport properties. The synthesis of compounds 1a/1b is outlined in Scheme 1. Compound 3a was prepared as already reported12 and compound 3b was synthesized by Suzuki coupling of 6-bromo-2-(trifluoromethyl)carbazole (2) with 1-naphtyl boronic acid in 89% yield. Compound 2 was obtained by treating 2-trifluoromethylcarbazole13 with NBS in the presence of LiClO4 and silica gel in ice-cooled CH2Cl2. Some multibromination was observed when the reaction was carried out at room temperature. Ullmann coupling of compounds 3a/3b with the respective 2,7-dibromo-9,9-dialkylfluorenes (1.5 equiv) led to bromo compounds 4a/4b in ca. 65% and 53% yields. Successive treatment of 4a/4b with n-butyllithium and 2-isoproxy4,4,5,5-tetramethyl-1,3,2-dioxaborolane in anhydrous THF at -78 °C afforded the corresponding Suzuki reagents 5a/ 5b. A bis-Suzuki coupling of 1,3,5-tribromobenzene with 2 equiv of 4,4,5,5-tetramethyl-2-p-sec-butoxyl-phenyl-1,3,2(13) Freeman, A. W.; Urvoy, M.; Criswell, M. E. J. Org. Chem. 2005, 70, 5014. 5319
Figure 2. Molecular structure of compound 1c.
Figure 1. UV-vis absorption and PL spectra of 1a/1b in p-xylene solution and as-cast films on quartz (PL excitation ) 380 nm).
dioxaborolane resulted in compound 6 in 86% yield, which underwent further lithiation and boronation reactions to yield Sukuki reagent 7. Precursor compound 8 was thus prepared in 78% yield by a mono-Suzuki coupling of 7 and 4,7-bromo2,1,3-benzothiadiazole. Finally, the target compounds 1a and 1b were obtained in ca. 93% yields by coupling a slight excess of precursor 8 with Suzuki reagents 5a and 5b, respectively. This high coupling efficiency greatly facilitated purification by routine column chromatography. Furthermore, the high solubility of compound 8 in ethanol and petroleum ether allows easy removal of the excess of reagent. The identity and purity of compounds 1a/1b was confirmed by 1H NMR, MALDI-TOF mass spectrometry, and microanalysis (see Supporting Information). These new compounds show a high solubility in common organic solvents such as CH2Cl2, THF, toluene, and p-xylene. For instance, 25 mg of 1a/1b is rapidly dissolved in 1 mL of p-xylene at room temperature without saturation.
The UV-vis and PL spectra of 1a/1b in dilute solution and as thin solid films are shown in Figure 1, and the relevant data are summarized in Table 1. Thin films of 1a and 1b spun-cast from p-xylene solutions show emission maxima at 518 and 521 nm, respectively. These highly green emissive solids show absolute solid-state PL efficiencies of ca. 0.72 for 1a and 0.51 for 1b, measured in an integrating sphere under 325 nm He-Cd laser excitation. Furthermore, comparison of the solid state PL spectra of 1a/1b to those of compound 1c (λem ) 568 nm, see Supporting Information) and 4,7-bis(9,9-diarylfluoren-2-yl)-2,1,3-benzothiadiazole (λem ) 544 nm)14 shows that the new compounds present a significantly improved green color purity, thus confirming the effectiveness of the 3,5-di(p-sec-butoxylphenyl)phenyl end-capping group to reduce intermolecular interactions. Cyclic voltammetry (CV) was performed in methylene chloride/acetonitrile in the presence of Bu4NPF6 as supporting electrolyte. Compounds 1a/1b are reversibly reduced at ca. -1.40 V vs Ag/AgCl (Figure 3). With respect to 1a, the introduction of CF3 substituent on the carbazole ring in 1b results in a substantial increase of the oxidation potential but leaves the reduction potential pratically unaffected. The LUMO energy levels estimated from the CV data of -2.93 eV for 1a and -2.94 eV for 1b seem appropriate for efficient electron injection (Table 1). The anodic peak potentials lead to estimated HOMO levels of ca. -5.61 and -5.84 eV for 1a and 1b, respectively, in good proximity to the values of -5.85 and -6.12 eV obtained by UPS measurements.15 Thermal gravimetry analysis shows that compounds 1a/ 1b are stable well above 350 °C. Differential scanning calorimetry (DSC) measurements on as-prepared samples reveal a distinct glass transition with Tg of ∼95 °C for 1a
Table 1. Summary of Optical and Electrochemical Data UV-vis (λabsmax, nm)a
1a 1b
PL (λemmax, nm, (quantum yield))
solution
solid
solutionb
solidc
reductiond(V)
oxidatione(V)
LUMOf (eV)
HOMO (eV)
302, 408 300, 400
285, 413 301, 413
504 (0.67) 500 (0.58)
518 (0.72) 521 (0.51)
-1.42 -1.41
1.26 1.49
-2.93 -2.94
-5.61g (-5.85)h -5.84g (-6.12)h
a In p-xylene. b ∼10-6 mol L-1 in p-xylene using quinine bisulfate in 0.1 mol L-1 H2SO4 as the reference (φr ) 0.54). c Measured in an integrating sphere under 325 nm laser excitation. d Derived from E0 values. e Derived from anodic peak potentials vs Ag/AgCl reference electrode. f Derived from E0 values. g Derived from anodic peak potentials with reference to the energy level of ferrocene of -4.8 eV vs vacuum level. h Measured by UPS.
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Figure 3. Cyclic voltammograms of 1a/1b in 5:1 CH2Cl2/CH3CN, supporting electrolyte 0.10 M n-Bu4NPF6, scan rate 50 mV s-1.
and ∼130 °C for 1b in the first heating run. Neither crystallization nor melting was observed upon heating up to 300 °C. The new compounds exhibit excellent film-forming properties. Atomic force microscopy images of spin-cast films of 1a and 1b from p-xylene solution show smooth, continuous, and featureless morphology with a surface root-meansquare roughness value of 0.35 and 0.42 nm, respectively (see Supporting Information). This stable homogeneous amorphous morphology appears favorable for light-emitting diodes with reduced leak currents and improved thermal stability during device operation. Compounds 1a/1b were first investigated as non-doped solution-processed emitters in single-layer diode structures ITO/PEDOT:PSS/1a(1b)/Ba/Al. The devices showed nearly pure green emission (λem ≈ 520 nm) with CIE coordinates (0.305-0.318, 0.60), very close to the European Broadcast Union color gamut green point CIE (x ) 0.29, y ) 0.60). Onset voltages of ∼2.8 and 3.3 V (at a luminance of 1 cd m-2) were determined for 1a and 1b, respectively. These low values suggest efficient charge injection into the emitting layer. Furthermore, this result also suggests that transport of at least one of the two types of charge carriers could be efficient. However the low device efficiencies (LEmax, ∼1 cd A-1) suggest unbalanced charge injection or poor charge confinement in the active layer. Consequently, double-layer devices consisting of an electron blocking PVK layer ITO/PEDOT:PSS/PVK/1a (1b)/ Ba/Al were fabricated. The device performance was greatly improved with a maximal luminous efficiency of 10.6 cd A-1 (corresponding to ηextmax of ∼3.7%16) for 1a and 7.5 cd A-1 (ηext ∼2.6%) for 1b, respectively. At a current density of ca. 20 mA cm-2, V ) 8.7 V, L ) 1228 cd m-2, LE ) 6.70 cd A-1, λmax ) 528 nm with CIE coordinates (0.344, 0.580) for 1a and V ) 9.0 V, L ) 1308 cd m-2, LE ) 6.18 cd A-1, λmax ) 528 nm with CIE coordinates (0.336, 0.577) for 1b. It is important to note that the device characteristics achieved here, especially without a hole-blocker, rank among the best reported so far for non-doped solution-processed
Org. Lett., Vol. 11, No. 22, 2009
Figure 4. L-V-J characteristics of devices ITO/PEDOT:PSS/PVK/ 1a(1b)/Ba/Al.
green fluorescent molecular emitters (see Supporting Information).17 These preliminary characterization of the singleand two-layer light-emitting devices also suggest that electron transport is facilitated in these new emitting materials. In summary, soluble green fluorescent molecular glasses based on 2,1,3-benzothiadiazole derivatives have been synthesized and characterized. These truly solution-processable molecular green emitters have been used as active materials in highly efficient green light-emitting devices. We have shown that the 3,5-di(p-sec-butoxylphenyl)phenyl moiety is an efficient end-cap block to alleviate intermolecular interactions, leading to green emission with improved color purity and PL efficiency. An in-depth analysis of the device operation, in particular, charge transport, is underway which should warrant further optimization of the device architecture and molecular structure. Acknowledgment. X.H.Z. is deeply grateful for the financial support of NSFC, MOE and MOST (Grant Nos. 50603006, 50911130174, 2006NCET, U0634003, 2009CB930601, and 2009CB623604). Ms. Chan Sun at Fudan University and Dr. Hai-Tao Xu and Prof. Xiang Zhou at Sun Yat-sen University are acknowledged with gratitude for the assistance in MALDI-TOF and UPS measurement. Supporting Information Available: Experimental and characterization detail of all new compounds. This material is available free of charge via the Internet at http://pubs.acs.org. OL9022563 (14) Ku, S. Y.; Chi, L.-C.; Hung, W.-Y.; Yang, S.-W.; Tsai, T.-C.; Wong, K.-T.; Chen, Y.-H.; Wu, C.-I. J. Mater. Chem. 2009, 19, 773. (15) D’Andrade, B. W.; Datta, S.; Forrest, S. R.; Djurovich, P.; Polikarpov, E.; Thompson, M. E. Organ. Electron. 2005, 6, 11. (16) ηext was verified by measurement in an integrating sphere. (17) (a) Zhou, Y.; He, Q.-Q.; Yang, Y.; Zhong, H. Z.; He, C.; Sang, G.-Y.; Liu, W.; Yang, C.-H.; Bai, F.-L.; Li, Y.-F. AdV. Funct. Mater. 2008, 18, 3299. (b) Liu, F.; Tang, C.; Chen, Q.-Q.; Li, S.-Z.; Wu, H.-B.; Xie, L.-H.; Peng, B.; Wei, W.; Cao, Y.; Huang, W. Org. Electron. 2009, 10, 256. (c) Lo, M. Y.; Zhen, C. G.; Lauters, M.; Jabbour, G. E.; Sellinger, A. J. Am. Chem. Soc. 2007, 129, 5808.
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